[csharp-netcore] Enum parameter serialization to use EnumMember value when provided.

[jafin]

PR relates to #11447

Change method ParameterToString (ClientUtils) to check if obj is Enum and has a EnumMember prop. If so use that value over the existing logic of obj.ToString()

This fixes an issue whereby the enum value does not match the value in the openApi schema.

This PR also modifies the location of the yaml location for csharp-netcore/OpenAPIClient-net5.0, to a cloned copy of the java petstore API with changes to trigger the scenario described here. This was done to avoid disturbing clients (java) unrelated to this change.

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[wing328]

C# (appveyor) tests passed via https://ci.appveyor.com/project/WilliamCheng/openapi-generator/builds/42576042

[jtreminio-dropbox]

This PR should have been merged months ago

[Ikelio]

Is there a way I can help resolving the conflicts ? Until this PR merges, any endpoint enum parameter which starts by a number, or a reserved word, will be generated incorrectly (that concerns a lot of API out there).

[wing328]

Sorry for the delay in reviewing this PR. There're too many PRs submitted this project.